Providing reliable high data rate services, e.g. real-time multimedia services, over wireless communication channels is a paramount goal in developing coding and modulation schemes. When a data rate for wireless communication systems is high in relation to bandwidth, multipath propagation may become frequency-selective and cause intersymbol interference (ISI). Multipath fading in wireless communication channels causes performance degradation and constitutes the bottleneck for increasing data rates.
Orthogonal frequency division multiplexing (OFDM) is inherently resistant to multipath fading and has been adopted by many standards because it offers high data-rates and low decoding complexity. For example, OFDM has been adopted as a standard for digital audio broadcasting (DAB) and digital video broadcasting (DVB) in Europe and high-speed digital subscriber lines (DSL) in the United States. OFDM has also been proposed for local area mobile wireless broadband standards including IEEE 802.11a, IEEE 802.11g, MMAC and HIPERLAN/2. Additionally, space-time (ST) multiplexing with multiple antenna arrays at both the transmitter and receiver are effective in mitigating fading and enhancing data rates. Therefore, multi-input multi-output (MIMO) OFDM is attractive for multi-user wireless communication systems. However, MIMO OFDM systems have increasing channel estimation complexity as the number of antennas increases due to the increased number of unknowns which must be estimated and have great sensitivity to carrier frequency offsets (CFO).
Typical single-input single-output (SISO) OFDM systems rely on blocks of training symbols or exploit the presence of null sub-carriers in order to acquire channel state information (CAI) to mitigate CFO and perform channel estimation. In the IEEE 802.11a, IEEE 802.11g, and HIPERLAN/2 standards, sparsely placed pilot symbols are present in every OFDM symbol and pilot symbols are placed in the same positions from block to block. Additionally, channel estimation is performed on a per block basis.
For channel state information (CSI) acquisition, three classes of methods are available: blind methods which estimate CSI solely from the received symbols; differential methods that bypass CSI estimation by differential encoding; and input-output methods which rely on training symbols that are known a priori to the receiver. Relative to training based schemes, differential approaches incur performance loss by design, while blind methods typically require longer data records and entail higher complexity. Although training methods can be suboptimal and are bandwidth consuming, training methods remain attractive in practice because they decouple symbol detection from channel estimation, thereby simplifying receiver complexity and relaxing the required identifiability conditions.